This invention relates to subwoofer audio speakers that produce low-frequency sound, generally operating below 100 Hz. The operating principle of a moving coil loudspeakers is based on coil of wire or (voice coil) immersed in a static magnetic field. This voice coil is fed electrical input signals and the resulting electromotive force induced in the coil forces the loudspeaker diaphragm to move.
“Subwoofer” is the term generally used today to refer to audio speakers that operate in the low frequency or bass range. Until more recent times, there was very little need to reproduce intense levels of sound down to 20 Hz (the low range for human hearing) because available programming sources were incapable of recording such frequencies. With the advent of more dynamic recording techniques, however, the ability of a subwoofer to reproduce low frequency input signals without distortion, i.e., as accurately as possible, has become a highly desirable objective in the industry.
Audio speakers, generally, are quite simply air pistons that move back (on the negative cycle of an electrical signal) and forth (on the positive cycle) creating different degrees of air pressure. These movements translate into different frequencies that, in turn, translate into Mozart or Manilow or Metallica at the human eardrum.
The typical subwoofer design has been around for over 50 years. Like any speaker, a subwoofer requires an audio amplifier which produces electrical impulses that alternate from positive to negative voltages and create an electromagnet when they reach a “voice coil” (a spool of wire) inside the speaker. The voice coil is suspended between the pole pieces of a permanent magnet motor structure. This voice coil is attached to the cone/moving mass assembly which is mounted to a frame that is fixed to the motor structure. The voice coil reciprocates (i.e., moves the cone back and forth) in a linear path as the alternating current flows through the voice coil that is centered in the magnetic gap (between the two poles). The circumference of the cone is affixed to a “surround” or “suspension”, which is affixed to a frame or basket, and which is generally constructed of a metal. The magnet is typically mounted to the rear of the frame behind the cone. The surround is generally a circular half-roll of flexible material that joins the top of the cone and the speaker's frame. A spider, which is a circle of flexible corrugated material, joins the bottom of cone to the speaker's frame. The surround and spider center the cone/moving mass assembly and restore it to its original position. The peak-to-peak distance traveled by the cone is known as the “excursion”. Generally, the peak-to-peak excursion of a conventional subwoofer is between 0.4–0.6 inches.
The voice coil movement causes the movement of the cone. Movement of the cone about the surround causes air to be moved, which is what produces the sound heard and, in the case of bass, felt by the listener.
The circular diaphragms or cones have been constructed of many different materials including paper, plastic and Kevlar™. Suspensions or surrounds are generally constructed of flexible, nonself-supporting, compliant materials such as relatively thin rubber, impregnated cloth, expanded synthetic cellular foam such as, e.g., expanded cellular polyethylene (“PE”) surround foam, or similar materials, which are compressible and produce very little resistance to peak-to-peak cone movement. The magnets are generally large ceramic magnets with a top plate, back plate and pole piece.
In a conventional speaker geometry, the large insulating ceramic magnets are located on the outside of the voice coil which does not allow for good thermal transfer away for from the coil. Because of the large size of a high power/high strength conventional voice coil motor, it often precludes being able to encapsulate the motor assembly. Conventional voice coil motors are typically mounted to the back of the frame due to similar reasons.
To achieve accurate low frequency reproduction, conventional subwoofers have been provided with long voice coils, large diameter voice coils, large magnets, large cones and large enclosures. There are, however, several limits in these design alternatives. There is, e.g., a practical limit on magnet size, design and weight. Activation of longer and larger voice coil results in large power losses in the form of heat. Possible thermal destruction of the coil imposes a limit on the power handling capacity of the speaker. Further, using large cones is problematic because it is difficult to design sufficiently rigid cones with large surface areas to resist distortion.
To reproduce high volume levels of low frequencies, a subwoofer must be capable of moving large quantities of air. It is possible to increase the excursion of the cone, and thus, increase the amount of air that is moved. However, when the excursion is increased, the efficiency of the speaker is substantially reduced, as less of the voice coil will remain in the magnetic gap. Another way to increase air movement is to use more than one subwoofer. This leads to large enclosures to house the devices.
Yet, today's market demands that a subwoofer be optimized for use in a relatively small enclosure, handle extremely high power levels, and displace a large volume of air all without a hint distortion.
Small enclosure compatibility requires a high moving mass, stiff suspension, low resonance, and high magnetic force. High thermal power handling requires large diameter voice coils and a means for dissipating the heat that is generated. To displace a large amount of air, the excursion must be as great a distance as possible. In other words, low distortion requires that the excursion be maximized and linear, the heat be minimized, and the electromechanical parameters be suited for small enclosures.
While many techniques have been used for removing heat from voice coils, and many additional techniques have been proposed, most of these techniques involve either active cooling through use of a fan or additional space-consuming hardware, such as a heat sink. A need therefore exists for a simple and relatively inexpensive technique for removing heat from the voice coil of a speaker, particularly a high performance speaker, which technique does not require either the use of an active cooling component or the use of an extra piece of heat dissipation hardware, but instead requires only the use of components normally existing in such a speaker.